Zeolite adsorbent for separation of para-xylene

ABSTRACT

Para-xylene may be recovered as raffinate by contacting a C-8 aromatic hydrocarbon mixture, in the presence of toluene desorbent, with a synthetic crystalline lithium aluminosilicate zeolite HP (formed by ion exchange from a sodium aluminosilicate zeolite HP having a lattice constant of 25.02-25.10 Å)--preferably in the presence of a pyridine.

FIELD OF THE INVENTION

This invention relates to solid-bed adsorptive separation. Moreparticularly, it relates to the separation of p-xylene from C-8 aromatichydrocarbon streams as a least strongly adsorbed, or front end raffinateproduct.

DESCRIPTION OF THE PRIOR ART

Solid bed adsorption techniques have been used to separate individualhydrocarbon isomers from charge hydrocarbon streams typified by C-8aromatic streams containing ethylbenzene and xylene isomers.

Separation of para-xylene from other charge streams has been describedin U.S. patents including:

    ______________________________________                                        3,558,730      3,761,533    3,960,774                                         3,558,732      3,795,711    3,997,620                                         3,626,020      3,855,333    4,029,717                                         3,663,638      3,878,127    4,031,155                                         3,686,342-3    3,878,129    4,051,192                                         3,696,107      3,894,109    4,069,172                                         3,734,974      3,943,183-4  4,313,015                                         ______________________________________                                    

In these illustrative patents, particular zeolites may be used toselectively adsorb para-xylene from feed mixtures which contain severalC-8 aromatic isomers; and in these patents the p-xylene is selectivelyadsorbed and is ultimately recovered as a tail-end or extract productwhile the remaining xylenes and ethylbenzene are recovered as front-endor raffinate components.

In other patents, typified by U.S. Pat No. 3,997,619, there aredisclosed processes for recovering ethylbenzene wherein this componentis relatively unadsorbed and is thus recovered as high purity front-endproduct, the xylene isomers being recovered as tail-end products - thisbeing effected by use of an adsorbent which is "all xylene" selective.

It is an object of this invention to provide a process for separatingp-xylene as front-end or raffinate product from a C-8 charge stream.Other objects will be apparent to those skilled in the art.

STATEMENT OF THE INVENTION

In accordance with certain of its aspects, this invention is directed toa process for separating para-xylene from a feed mixture containing C-8aromatic hydrocarbons including para-xylene which comprises contactingsaid feed mixture with, as an adsorbent, lithium high pressure zeolite,formed by ion exchange from a sodium aluminosilicate zeolite HP having alattice constant of 25.02-25.10 Å and a ratio of silicon atoms toaluminum atoms in the unit cell below 1.0, thereby selectively adsorbingsubstantially all of said C-8 aromatic hydrocarbons to the substantialexclusion of para-xylene; and recovering said para-xylene as a raffinatestream.

DESCRIPTION OF THE INVENTION

The charge mixtures which may be treated by the process of thisinvention include mixtures containing C-8 aromatic hydrocarbonsincluding para-xylene. These mixtures, which contain substantialquantities of ethylbenzene and the xylene isomers, generally areproduced by reforming and isomerization processes which are well knownto the refining and petrochemical arts. In reforming processes, anaphtha feed may be contacted with a platinum-halogen-containingcatalyst at severities selected to produce an effluent containing C-8aromatic isomers. Generally the reformate is then fractionated toconcentrate the C-8 aromatic isomers in a C-8 fraction. The C-8 aromaticisomers may then be further concentrated by solvent extractionprocesses. Xylene isomerization processes isomerize at isomerizationconditions a xylene mixture which is deficient in one or more isomers togive an effluent containing approximately equilibrium quantities of theC-8 aromatic isomers. The equilibrium composition of the xylene isomersand ethylbenzene at various temperatures are shown in the Table below.

                  TABLE                                                           ______________________________________                                        EQUILIBRIUM C-8 AROMATIC COMPOSITIONS*                                                 Temperature °F.                                                        620        800     980                                                        Mole percent of isomers                                              ______________________________________                                        Ethylbenzene                                                                              6            8      11                                            Para-xylene                                                                              22           22      21                                            Meta-xylene                                                                              50           48      45                                            Ortho-xylene                                                                             22           28      23                                            ______________________________________                                         (*based on API sources)                                                  

Feed streams may contain ethylbenzene and any of the xylene isomers inaddition to para-xylene. Extracted C-8 reformate fractions andisomerates from xylene isomerization processes containing all of thexylene isomers can be charged as feed streams. Feed streams includeeffluent streams from processes which have removed varying amounts ofone or more xylene isomers or ethylbenzene. As an example, at least aportion of the ortho-xylene may have been previously removed byfractionation from a feed mixture containing the xylene isomers.Ortho-xylene has a boiling point of about 6° F. higher than that of thenearest other C-8 aromatic (meta-xylene) and hence can be removed as abottoms product from ortho-xylene fractionator towers. The concentrationof ortho-xylene in the effluent from this fractionation process whichcan be used as a feed stream may be less than the concentrations ofeither para-xylene or meta-xylene.

Ethylbenzene, which has a lower boiling point than any of the xyleneisomers, may also be separated by distillation, preferably after removalof at least a portion of the ortho-xylene. The concentration ofethylbenzene in the effluent from this fractionation process which canbe used as a feed stream may be less than the concentrations of eitherpara-xylene or meta-xylene. Removal of ethylbenzene and/or ortho-xylenefrom C-8 aromatic mixtures may be effected by distillation.

C-8 aromatic components, other than those desired as product, should bepresent in the feedstock at as low a concentration level as possible.Thus for para-xylene production, the content of meta-xylene,ortho-xylene, and ethylbenzene should be as low as possible. Forproduction of both para-xylene and meta-xylene, it is desirable tomaintain the content of ortho-xylene and ethylbenzene as low aspossible. In practice, only ortho-xylene and ethylbenzene can be removedby distllation, so a charge stock containing a concentrate ofmeta-xylene and para-xylene would be typically available for productionof either para-xylene or para-xylene and meta-xylene. It is to be notedthat separation of ethylbenzene by distillation is expensive; andaccordingly economic considerations may dictate that the feedstock wouldhave been treated in a manner to principally reduce the content ofortho-xylene.

In accordance with practice of the process of this invention, the feedmixture containing C-8 aromatic hydrocarbon including para-xylene may becontacted with, as an adsorbent, a lithium zeolite HP, therebyselectively adsorbing substantially all of said C-8 aromatic hydrocarbonto the substantial exclusion of para-xylene.

The synthetic crystalline lithium aluminosilicate zeolite HP adsorbentswhich may be employed in practice of the process of this invention mayinclude those prepared by ion exchange from sodium HP zeolite which aremade at high pressures at moderate temperatures and which areparticularly characterized by a lattice constant of 25.02-25.10 Å and bya ratio of silicon atoms to aluminum atoms of below 1.0. Typical of thelithium HP zeolites may be those prepared by lithium-exchanging the NaHPzeolites of U.S. Pat. No. 4,289,740 which issued Sept. 15, 1981 toTexaco Inc. as assignee of John H. Estes, or U.S. Pat. No. 4,306,962which issued Dec. 22, 1981 to Texaco Inc. as assignee of John H. Estes.

The synthetic crystalline sodium aluminosilicate zeolite HP (NaHPzeolite) may be prepared by forming an aqueous solution containingsodium aluminosilicate (Na₂ O--SiO₂ --Al₂ O₃) in amounts and ratiosufficient to yield a product zeolite having a ratio of silicon atoms toaluminum atoms of below 1.0, typically 0.8-1.0.

The mixture may preferably be aged at pressure above 20,000 psig. Morepreferably aging is carried out at pressure above 40,000 psig. Althoughaging may be carried out at 20,000-80,000 psig, it is preferablyeffected at 20,000-60,000 psig, say 40,000-50,000 psig, commonly about50,000 psig. Aging may be carried out at room temperature up to 100° F.,preferably 70° F. for 8-24 hours, preferably 16 hours.

The mixture, preferably after aging, is subjected to HP zeolite-formingpressure of 20,000-80,000 psig, preferably 20,000-60,000 psig, say40,000-50,000 psig, commonly about 50,000 psig. Temperature of operationmay be 150° F.-350° F., preferably 150° F.-250° F., say 200° F. over8-16 hours, say 8 hours. In the preferred embodiment, the pressure willbe the same in the aging step (if and when employed) as it is in thesubsequent reaction step.

The typical NaHP zeolites obtained by recovery of the product from thereaction may have the following formula:

    Na.sub.a [(A10.sub.2).sub.a (SiO.sub.2).sub.b ]c H.sub.2 O

In this formula a plus b is 192. a is greater than 96 and preferably97-108. In the formula, c is commonly 264.

A typical NaHP zeolite may have the formula:

    Na.sub.103 [(A10.sub.2).sub.103 (SiO.sub.2).sub.89 ]. 264 H.sub.2 O

The NaHP zeolite so-prepared is commonly typified by a ratio of siliconatoms to aluminum atoms of below 1.0, preferably 0.8-1, say 0.98 and bya lattice constant of preferably 25.02-25.10 Å, say 25.08 Å.

Conversion of the NaHP zeolite to the LiHP zeolite may be effected byimmersing the former in an excess of an aqueous solution of awater-soluble salt of lithium--preferably lithium acetate, lithiumformate, etc. at preferably ambient temperature of 60° F.-190° F., say160° F. for 0.5-4 hours, say 1 hour. The solution may be removed and theprocedure repeated 3-5, say 5 times, with washing with distilled waterbetween each exchange.

Typical LiHP zeolites may include

    Li.sub.103 [(A10.sub.2).sub.103 (SiO.sub.2).sub.89 ]264 H.sub.2 O

In practice of the process of this invention according to certain of itsaspects, adsorption is effected with a lithium high pressure zeolitecontaining pyridine se or a substituted pyridine selected from the groupconsisting of 1-picoline, 3-picoline, 4-picoline, 2,4-lutidine,2,6-lutidine, and 3,4-lutidine. Although it may be possible to obtainseparation of para-xylene by the use of other pyridines, such as thepicolines (including 2-picoline or 3-picoline, or 4-picoline), or thelutidines (including 2, 4-lutidine or 2,6-lutidine, or 3,4-lutidine), itis found that most effective operation may be achieved by the use ofpyridine se.

The organic selectivity modifier (preferably pyridine) may be containedin the zeolite in amount of about 10%-60% of the total adsorptivecapacity (C-8 aromatic plus modifier) of the zeolite. The capacity ofLiHP-type zeolite, expressed as weight % of adsorbed component(s)relative to weight of dry adsorbent, may typically be 10-25 with manyfalling in the 13-23 range. Total capacity may be essentially equivalentfor C-8 aromatics, pyridine, and their mixtures. The pyridine loading istypically in the range of 1-20 wt. %, preferably 3-8 wt. %, say about 5wt. % of the amount of zeolite adsorbent.

Thus, the weight ratio of pyridine to total capacity of the zeolite maybe of the order of 0.1-0.6 or more, say 0.2-0.5.

A typical instance may utilize a pyridine loading of 4.8% (of theadsorbent) where the adsorbent may have a total capacity of 18.5%corresponding to a ratio of 0.26.

In one embodiment, the pyridine is loaded onto the zeolite adsorbentprior to initiation of operation; and this may commonly be effected bycontacting the pyridine modifier with the adsorbent before the latter isadmitted to the reaction vessel. Preferably, the modifier is mixed with,or dissolved in, the desorbent to be used in the process as hereinafterdescribed and the adsorbent is submerged in the mixture. At roomtemperature, pyridine is substantially completely removed from solutionby the zeolite.

The adsorbent can be contained in one or more chambers where throughprogrammed flow into and out of the chamber separation of the isomers iseffected. Preferably in operation, fixed quantities of a charge streamand of a desorbent stream (both preferably anhydrous) are admittedalternately to one end of a bed or column of zeolite; and effluent fromthe other end of the column is segregated into cuts. The bed may beoperated in either up-flow or down-flow mode. Concentrations ofindividual charge components and of desorbent in effluent from thecolumns resulting from this operation vary with time (or quantity oftotal effluent). The resolution of components taking place in the columnis characterized as a cyclic, chromatographic, adsorptive separationwhere the cycle time is the interval between the start of introductionof corresponding successive portions of charge (or of desorbent) to thecolumn, or their appearance in the effluent. Effluent from the columnduring each cycle is segregated into fractions, or cuts, which mayinclude (1) a front end or raffinate cut or product cut taken at thebeginning of the cycle in which the least strongly adsorbed chargecomponent (para-xylene) is concentrated to high purity relative to othercharge components; (2) one or more intermediate cuts in which the frontend product component is concentrated relative to other chargecomponents, but at a lower purity level than in the front end cut (suchcuts may be recycled to the charge preparation operation to permitsubstantially complete recovery of product component(s) in high purity);and (3) one or more cuts in which only small amounts of productcomponent(s) are present. If desired, cut(s) (2) may be combined withcut(s) (3).

The cyclic process may be carried out either in the liquid phase or inthe vapor phase. Liquid phase operation may be carried out at lowertemperatures and may permit easier control of charge and cut points, butvapor phase operation is preferred because of the much greaterseparation efficiency afforded by this mode. Preferred conditions forthe process of this invention in liquid phase operation will includetemperatures within the range from about 100° to about 450° F. atpressures sufficient to maintain a liquid phase and to provide a drivingforce for moving fluid through the adsorbent bed, generally in the rangefrom about atmospheric to about 500 psig. Preferred conditions for theprocess of this invention in vapor phase operation will includetemperatures from about 290° F. to about 450° F. sufficient to maintaincomponents in the vapor phase at pressures from about atmospheric toabout 80 psig, the pressure preferably being the minimum required todrive fluid through the system.

In both liquid and vapor phase modes, operation is substantiallyisothermal; and pressure drop across the system is substantiallyconstant, although some variation may occur during the course of acycle. The quantity of desorbent introduced for a given quantity ofcharge is sufficient to displace all charge components to an extent thatthe residual total charge component concentration in the effluent for agiven cycle is very low, preferably below about 0.1%, before chargecomponents from the following cycle start to appear. This determines theminimum preferred desorbent: charge ratio; if less desorbent is used,product purity in subsequent cycles is reduced. If more desorbent isused, separation is still achieved, but the cycle time and amount ofdesorbent to be removed from product fractions are unnecessarilyincreased. The quantity of charge introduced per cycle and the minimumdesorbent: charge ratio for this quantity of charge are related to anumber of factors including adsorbent capacity, selectivity, andparticle size, fluid flow rate, and particularly to charge compositionand to column length. Preferred process design specifications arelargely related to the cost of the absorbent bed per unit of pureproduct production rate and to the cost of separating desorbent fromeffluent fractions; both costs must be considered together.

The process of this invention may also be effected in a simulated movingbed countercurrent system. The operating principles and sequence of sucha flow system are described In U.S. Pat. No. 2,985,589 issued to D. B.Broughton which patent is incorporated herein by specific referencethereto. This system may be operated in the liquid phase mode with thesame zeolite HP adsorbents and in the same temperature and pressureranges as those previously described for cyclic operation in the liquidphase mode. Para-xylene is recovered as a least strongly adsorbed, orraffinate product. Operation of a simulated moving bed countercurrentsystem in the vapor phase mode, while possible in principle, would bedifficult to achieve in practice; so if the advantageous separationefficiency of the vapor phase mode is to be obtained, the cyclicoperating procedure is preferred.

The desorbent materials which are used in the preferred processingschemes employed may vary depending on the type of operation employed.The term "desorbent material" as used herein means any fluid substancecapable of removing a selectively adsorbed feed component from theadsorbent. In the swing-bed system, in which the selectively adsorbedfeed component is removed from the adsorbent by a purge stream,desorbent materials comprising gaseous hydrocarbons such as methane,ethane, etc., or other types of gases such as nitrogen or hydrogen maybe used at elevated temperatures or reduced pressures or both toeffectively purge the adsorbed feed component from the adsorbent.

However, in adsorptive separation processes which employ zeoliteadsorbents and which are generally operated at substantially constantpressures and temperatures, the desorbent material relied upon must bejudiciously selected to satisfy several criteria. First, the desorbentmaterial must displace the adsorbed feed component from the adsorbentwith reasonable mass flow rates without itself being so stronglyadsorbed as to unduly prevent charge components from displacing thedesorbent material in a following adsorption cycle. Secondly, desorbentmaterials must be compatible with the particular adsorbent and theparticular feed mixture. More specifically, they must not reduce ordestroy the critical selectivity of the adsorbent for the components ofthe charge.

Desorbent materials to be used in the process of this invention shouldadditionally be substances which are easily separable from the feedmixture that is passed into the process. Each of the effluent cuts incyclic processes, and both raffinate and extract streams in simulatedmoving bed countercurrent processes, contain desorbent in admixture withcharge components. Without a method such as distillation, for separatingdesorbent material the product purity would be low; and consumption ofdesorbent in the process would be excessive. Any desorbent material usedin this process will have a substantially different average boilingpoint from that of the feed mixture. The use of desorbent materialhaving a substantially different average boiling point than that of thefeed allows separation of desorbent material from feed components in thevarious effluent cuts or the extract and raffinate streams byfractionation thereby permitting reuse of desorbent material in theprocess. The term "substantially different" as used herein means thatthe difference between the average boiling points between the desorbentmaterial and the feed mixture shall be at least 15° F. The boiling rangeof the desorbent material may be higher or lower than that of the feedmixture.

Among the desirable characteristics of an adsorbent are: adsorptivecapacity for some quantity of an extract component per unit quantity ofadsorbent; the selective adsorption of feed components with respect toone another such that a desired pure product component is adsorbed morestrongly or less strongly than the other components; and sufficientlyfast rates of adsorption and desorption of the extract components to andfrom the adsorbent.

Capacity of the adsorbent for adsorbing components of the separationsystem, including desorbent, is, of course, a necessity; without suchcapacity the adsorbent is useless for adsorptive separation. Increasedcapacity of a particular adsorbent makes it possible to increase theseparation efficiency and thereby reduce the amount of adsorbent neededto effect separation of a particular feed mixture at a given productpurity and yield. (Yield is defined as the fraction of a feed componentrecovered as pure product.) A reduction in the amount of adsorbentrequired for a specific adsorptive separation reduces the cost of theseparation process. It is important that the good initial capacity ofthe adsorbent be maintained during actual use in the separation processover some economically desirable life.

The second necessary adsorbent characteristic is the ability of theadsorbent to separate components of the feed; or, in other words, thatthe adsorbent possess adsorptive selectivity for one component ascompared to another component. Relative selectivity can be expressed notonly for one feed component as compared to another but can also beexpressed between any feed mixture component and the desorbent material.The selectivity as used throughout this specification is defined as theratio of concentrations of the two components in the adsorbed phasedivided by the ratio of concentrations of the same two components in theunadsorbed phase at equilibrium conditions.

Determining these adsorbent characteristics, particularly capacity andselectivities for charge and desorbent components, is essential fordeveloping an adsorptive separation system for recovering specific purecomponents from mixtures with difficulty separable substances, such asisomers of the desired products. Further, once such a system isestablished, a convenient test method is required for determining thatsubsequent batches of adsorbent are equivalent to the original adsorbentor fall within a satisfactory range. I have found a convenient andeffective procedure for accomplishing these objectives which comprisesthe steps of:

(1) Combining a suitably prepared (i.e. dried to a specific moisturelevel) adsorbent sample with a test mixture of test components, whichmay include components of a mixture to be separated, desorbentmaterials, and adsorbent modifiers, in the presence of a referencecomponent which is essentially unadsorbed and essentially inert, in thepresence of strongly adsorbed test components. For determiningadsorption equilibria for mixtures of aromatic hydrocarbons, paraffinicor cycloparaffinic hydrocarbons are suitable reference components. Aparticularly suitable reference component is cyclohexane.

(2) Equilibrating the solid-liquid mixture with suitable agitation in asealed vessel at a convenient temperature, which may be roomtemperature.

(3) Separating equilibrated liquid from solid adsorbent (e.g. bycentrifuging), sampling the liquid, and analyzing the liquid by asuitable procedure (e.g. gas chromatography) for determining theconcentration of each of the components present.

(4) From the known weight and composition of the testcomponent-reference component test mixture charged, and composition ofequilibrium liquid, calculating the quantity of each test component inthe equilibrium liquid using as a basis for such calculation theoriginally charged weight of inert (non-adsorbed) reference component inboth the test liquid and equilibrium liquid.

(5) By difference, from the calculated weight of each component in theequilibrium liquid and the known amount of each component in the charge,calculating the weight of each test component adsorbed.

(6) From the weight of adsorbent charged and the weights of testcomponents adsorbed, calculating the capacity of the adsorbent for thetest components and the composition of the adsorbed phase.

(7) From the calculated composition of the adsorbed phase and thecomposition of the equilibrium liquid phase obtained by analysis,determining selectivity of the adsorbent for any pair of testcomponents.

The method may be used to screen separation systems prior to columnoperation; separations obtained from column operation at elevatedtemperatures are found to correspond to those expected from theadsorbent characteristics determined by the test method. It may be usedto determine variation of selectivity with fluid phase composition, arelationship not readily obtainable from other methods for estimatingadsorbent selectivities. It may be used to determine the effects ofcomponents added to modify the selectivity characteristics of originaladsorbents or to determine the effects of impurities (such as water)which may be present in charge or desorbent streams, particularly incommercial operation. It may be used to determine variations ofadsorbent capacity, which may be due to occluded solid material in thepores or to variations in the quantity of binder used, which do notappreciably affect adsorbent selectivity. It may be used as an adsorbentspecification test where specific values or ranges of capacity andselectivity for specific test components at particular concentrationlevels are specified. It may be used to select suitable desorbentmaterials. It may also be used as a control test during manufacture ofzeolites.

The preferred desorbent may be toluene. Benzene may be employed asdesorbent.

In isothermal, isobaric, operation of the process of my invention, Ihave found that desorbent materials comprising mono-cyclic-aromatichydrocarbons are particularly effective. Specifically, desorbentmaterials comprising toluene are preferred for this type of operation.

In operation of the process of this invention in the cyclic, liquidphase mode, the LiHP zeolite (optionally pyridine-loaded) packed in theadsorption column, is flooded with desorbent e.g. toluene which ispassed downwardly through the adsorbent bed at a flow rate of 0.1-6, sayabout 2 gallons per minute per square foot of column cross section.Periodically the flow of toluene is interrupted and a portion of chargeis introduced at about the same flow rate. Pyridine, if used, ispreferably added with the desorbent toluene to balance pyridine removedfrom the adsorbent bed in the column effluent; the amount added dependson the pyridine loading of the adsorbent and the temperature of theadsorption column--typically it may be 0.01%-1.0%, say 0.2% of thedesorbent toluene. As the effluent is monitored (by gas chromatography,for example), toluene desorbent is first observed. When the first C-8component shows, which in this process is para-xylene, the cycle isconsidered started.

It is possible to collect incremental portions of product (over equaltime increments), but preferred operation is carried out by collectingthe entire yield of high purity (i.e. 99+% purity) para-xylene in onealiquot. Depending on the details of the downstream processing facility,there may be recovered a second product stream which is rich inpara-xylene although it is of a purity less than that of the firstproduct stream. A third stream may be recovered which contains a mix ofC-8 components.

Each of these product streams may be separated, as by distillation, fromthe toluene and pyridine. Pyridine and toluene are recovered togetherand may be recycled.

Operation in the vapor phase is comparable. The absorbent may be loadedwith pyridine, if used, and toluene in liquid phase ab initio. Thesystem is then heated to e.g. 340° F. and liquid displaced from thecolumn by passing vapor phase toluene desorbent (containing addedpyridine) downwardly through the adsorbent bed. Charge is introducedperiodically in the vapor phase. The effluent is condensed and collectedin desired increments followed by recovery of the desired high puritypara-xylene from toluene and pyridine in the front end product.

ADVANTAGES OF THE INVENTION

It is a feature of the process of this invention that it ischaracterized by many advantages including the following:

(i) it permits attainment of p-xylene as a front-end product which istypically more easily purified than is the tail-end product, and whichmay be recovered as product having a very low level of C-8 impurities;

(ii) it permits operation under conditions of high selectivity;

(iii) selectivity increases as the concentration of less stronglyadsorbed component increases, and thus operation at the front-end of theadsorption cycle permits higher selectivity to be realized as theproportion of p-xylene in C-8 aromatics increases in this portion of thecycle; and

(iv) the content of impurities, originating from the tail of a precedingcycle, is measured against a high front-end product peak and, even underthe least favorable conditions of operation, causes a smaller loss ofproduct purity than would the internal front end components tailing intoa back end product

DESCRIPTION OF THE DRAWINGS

The drawing shows a schematic process flow sheet by means of which a C-8hydrocarbon may be treated according to a preferred embodiment of thisinvention.

DESCRIPTION OF PREFERRED EMBODIMENTS

Practice of the novel process of this invention may be apparent to thoseskilled in the art from the following description of various embodimentswherein as elsewhere in the description, all parts are parts by weightunless otherwise specified.

EXAMPLES I-1 TO I-14

In these examples, equilibrium data were obtained for adsorption of C-8aromatic isomers on various HP zeolite samples both in their originalform and containing pyridine.

Adsorbents used were dried in a tube furnace under a flow of drynitrogen, and then were loaded into previously weighed glass ampoules ina nitrogen-flushed dry box. The weight of adsorbent was determined, andthen a quantity of previously prepared charge mixture was introduced,the ampoule was chilled in liquid nitrogen, evacuated, sealed, andweighed to determine the weight of charge mixture. Charge mixtures weremade up with known amounts of C-8 aromatics, inert reference component(cyclohexane), and in designated instances, pyridine. The ratio ofreference component to total C-8 aromatics (and desorbent, if present)was generally 1.0-1.5:1. Pyridine (when present) was present in anamount sufficient to give a desired ratio of pyridine/dry adsorbent. Theweight ratio of liquid charge to adsorbent was normally about 2 and thequantity of adsorbent used was generally about 1 gram.

Loaded ampoules were agitated on a shaker table at a temperature ofabout 75° F. for an equilibration period. Typically the equilibrationperiod was about one week up to several months, but equilibration wasprobably substantially complete after one to two days. Afterequilibration, ampoules were usually centrifuged to facilitateseparation of liquid from adsorbent, then the liquid was sampled andanalyzed by gas chromatography. Replicate analyses were usually obtainedand analyses of samples of charge liquid (retained in sealed ampoules)were usually carried out at the same time.

The quantity of each C-8 aromatic (and desorbent, if present) in theequilibrium liquid was calculated from the GC analysis and known weightof reference component in the charge as ##EQU1## The correspondingquantity of component adsorbed per unit weight of adsorbent charged wascalculated as: ##EQU2## Pyridine loading of the adsorbent (per unitweight of adsorbent) was calculated on the basis of complete pyridineadsorption as: ##EQU3## The gas chromatographic procedure used forsample analysis was not suitable for pyridine analysis, but measurednitrogen concentrations indicate that the amount of pyridine in theequilibrium liquid is negligible.

Total capacity of the adsorbent is the sum of individual componentadsorption values plus the pyridine loading. Composition of the C-8aromatics (and desorbent, if present) in the adsorbed phase iscalculated from the individual component adsorption values.

The selectivity with which an adsorbent adsorbs one component relativeto another is a measure of its separation capability. Selectivityfactors alpha (α) are commonly used as measures of adsorbent selectivitybetween components of a mixture. Selectivity between any two componentsis defined as: ##EQU4## Thus α will be greater than 1.0 if component 1is more strongly adsorbed than component 2. With a multi-componentmixture it is convenient to express selectivities of the componentsrelative to a particular component of the mixture.

Selectivity factors in the present examples are calculated from thecomposition of the adsorbed phase, determined as described above, andthe composition of the equilibrium liquid determined by gaschromatography.

In each of Examples I-1 to I -14, a charge liquid containing equalportions by weight of ethylbenzene, p-xylene, m-xylene, and o-xylene(plus cyclohexane reference component) was equilibrated against adesignated HP zeolite. When pyridine was present, it is reported asweight percent of adsorbent.

The following table sets forth equilibrium data at 75° F. on various HPzeolites--both with and without pyridine modifier. The Pyridine Loadingis expressed as weight percent of the dry adsorbent, as charged.Capacity of the zeolite (total and C-8) is expressed as weight %adsorbed component(s) based on the weight of adsorbent.

There are tabulated the selectivity (alpha) of (i) EB (ethylbenzene)with respect to p-xylene; (ii) P-X (para-xylene) with respect top-xylene, which is of course 1.00, included for reference; (iii) M-X(meta-xylene) with respect to p-xylene; and (iv) O-X (ortho-xylene) withrespect to p-xylene.

                  TABLE                                                           ______________________________________                                        EQUILIBRIUM DATA - HP ZEOLITES                                                WITH AND WITHOUT PYRIDINE MODIFIER (75° F.)                                           HP                                                             Ex-   Pyridine Zeo-   Capacity Selectivity                                    ample Loading  lite   Total C-8  EB   P    M    O                             ______________________________________                                        I - 1*    0        Na   20.7       1.38 1.00 1.00 0.99                            2*    0        K    15.4       1.46 1.00 0.49 0.74                            3*    0        La   16.4       1.01 1.00 1.31 2.46                            4     0        Li   21.0       1.30 1.00 1.23 1.95                            5     0        Li   21.1       1.29 1.00 1.22 1.93                            6     0        Li   22.9       1.41 1.00 1.47 1.93                            7     0        Li   22.2       1.41 1.00 1.45 1.88                            8*    0        Mg   10.8       1.06 1.00 1.10 1.41                            9*    0        Zn    7.1       0.88 1.00 1.20 1.76                            10    3.1      Li   20.6  17.4 1.99 1.00 1.98 2.00                            11    3.0      Li   18.5  15.5 2.28 1.00 2.30 2.29                            13    3.0      Li   21.3  18.4 2.37 1.00 2.50 2.37                            14    5.7      Li   21.8  16.0 3.25 1.00 3.73 2.67                        ______________________________________                                    

From the above table, it will be apparent to those skilled in the artthat the novel technique of this invention permits attainment ofraffinate containing substantially pure para-xylene. For example, ofthose runs without pyridine, control Example I-1* using NaHP showedselectivity of M-X equal to that of P-X and thus no separation ispossible. The selectivity of O-X is almost identical; and thus thisisomer may not be separately recovered.

In the case of control Example I-2*, using KHP zeolite, the para isomeris undesirably found in the intermediate stream. Similar results areattained using ZnHP zeolite in control Example I-9*. In control ExampleI-3*, using LaHP, the separation between EB and P-X is minimal. Incontrol Example I-8* using MgHP, the separation between EB and P-X andbetween M-X and P-X are both minimal.

In the case of the pyridine-containing systems (Examples I - 10 through14), it is apparent that it is possible to recover a pure para-xyleneraffinate stream by the use of LiHP zeolites. A comparison of thosepreferred Examples with less preferred Examples I - 4 through 7 (whichlatter did not utilize pyridine) show that the presence of pyridinegives a greater separation of selectivities between P-X on the one handand the other isomers on the other. Thus, although it is possible toeffect separation of a raffinate containing para-xylene by use of LiHPzeolite containing no pyridine, improved results may be achieved in thepresence of pyridine.

Results comparable to Examples I-4 through I-7 or I-10 through I-14 maybe attained in practice if the desorbent is:

                  TABLE                                                           ______________________________________                                        Example               Desorbent                                               ______________________________________                                        II                                                                            1                     benzene                                                 2         toluene                                                             3         p-diethylbenzene                                                    ______________________________________                                    

Results comparable to Examples I-10 through I-14 may be attained if thesubstituted pyridine is:

                  TABLE                                                           ______________________________________                                        Example               Pyridine Modifier                                       ______________________________________                                        III                                                                           1                     2 - picoline                                            2          3 - picoline                                                       3          4 - picoline                                                       4          2,4 - lutidine                                                     ______________________________________                                    

EXAMPLE IV

In this Example there is set forth the best mode known to me at thistime for practicing the process of this invention. The drawing shows aschematic process flow sheet of this embodiment of the process.

In this embodiment, the charge C-8 stream from which it is desired torecover para-xylene contains

    ______________________________________                                        ethylbenzene   20.0 w %                                                       para-xylene    20.3 w %                                                       meta-xylene    39.7 w %                                                       ortho-xylene   20.0 w %                                                       ______________________________________                                    

This stream is admitted through line 10 to disillation operation 11wherein there is separated pure ortho-xylene, recovered through line 12.

The composition in line 13 (1000 parts) typically may contain

    ______________________________________                                        ethylbenzene   24.7 w %                                                       para-xylene    25.0 w %                                                       meta-xylene    49.0 w %                                                       ortho-xylene    1.3 w %                                                       ______________________________________                                    

Adsorption operation 14 utilizes LiHP zeolite which has been loaded with5 w% of pyridine. The outlet of the column is at atmospheric pressure.Toluene desorbent containing pyridine (in amount sufficient to prevent anet loss of pyridine during the course of the cyclic operation), isadmitted in the vapor phase at 340° F. through line 15 to the top of thecolumn and passed through the adsorption bed at a flow rate such thatthe quantity of toluene introduced per unit time per unit cross sectionof adsorbent column is equivalent to 0.5 gallons of liquid toluene(measured at room temperature) per minute per square foot of columncross section.

Periodically, the flow of toluene vapor (in this example, all referencesto toluene from line 15 refer to toluene containing pyridine) to thecolumn from line 15 is interrupted and charge, in the vapor phase at340° F., is admitted through line 13 to the top of the column and ispassed through the adsorption bed at a flow rate such that the quantityof charge introduced per unit time per unit cross section of adsorbentcolumn is equivalent to 0.5 gallons of liquid charge (measured at roomtemperature) per minute per square foot of column cross section. Theflow of charge C-8 hydroarbon alternates with the flow of toluene fromline 15.

Alternate introduction of charge and toluene desorbent is continued, theinterval between sequential introductions of charge (or of toluene)comprising a single cycle. Toluene is introduced during each cycle. Alarger quantity of toluene per cycle may be used without affectingproduct quality, but at the expense of greater cycle time and greatercost for separating desorbent from C-8 aromatic components. Use of asmaller quantity of toluene per cycle reduces the yield of pure product.

A single complete cycle is considered here to comprise the effluent fromthe point where total C-8 aromatic content rises above 0.1 wt. % to thepoint where it falls below 0.1 wt. %. In each cycle, in which 1000 partsof charge are introduced through line 13 a first cut is taken throughline 16 starting at the point where C-8 aromatic concentration reaches0.1%. This first cut contains 145.4 parts of para-xylene to theexclusion (about 0.45 w% of the para-xylene) of other C-8 isomers, plustoluene containing pyridine. This mixture is separated by distillationin distillation operation 17 to permit recovery in line 18 of tolueneand pyridine which may be recycled to line 15 with or withoutintermediate separation or purification. There is recovered in line 19,146.1 parts of 99+ wt. % para-xylene.

The second cut, recovered in line 20, contains 196.3 parts of C-8isomers of which about 51 w% is para-xylene, together with toluene andpyridine. This mixture is separated in distillation operation 21 topermit recovery in line 22 of toluene and pyridine which may be handledin manner similar to that for the comparable stream in line 18. Thestream in line 23 (196.3 parts) is a para-xylene stream containing 51 w% para-xylene together with other C-8 isomers. This latter cut may berecycled, as to line 10. In the simplest mode of operation, it may bedesirable to not separately recover this second cut, but to combine itwith the third cut infra.

The third cut recovered in line 24 contains little of no para-xylene inthis embodiment. Typically it contains 657.6 parts of C-8 components ofwhich 0.7% is para-xylene, and toluene containing pyridine. This mixtureis separated by distillation in distillation operation 25 to yield inline 26, toluene containing pyridine, this stream being handled inmanner similar to streams 18 and 22, either separately or in combinationtherewith. There may be recovered in line 27 a C-8 stream containing inthis embodiment 0.7% of para-xylene.

It will be apparent to those skilled in the art that this processingscheme may be modified depending on the concentration of the severalcomponents in the charge or upon the needs of the processor. Forexample, it may be desirable to recover the second and third cutstogether rather than separately. The para-xylene-rich stream recoveredby distillation of the second cut may be recycled to charge in line 10or passed to a separate separation system, etc.

EXAMPLE V

In this example which sets forth a procedure for preparing LiHP zeolitesfrom NaHP zeolites, 10 grams of NaHP zeolite (having a lattice constantof 25.04 Å and a silicon to aluminum ratio of 0.96) were exchanged fivetimes using 100 cc of lN lithium acetate. Each exchange was carried outat 160° F. for 1 hour. The product was washed, between exchanges, with100 cc of distilled water and with 200 cc of distilled water after thefinal exchange. Analysis showed Li₂ O 9.67% and Na₂ O 2.07%. X-ray wascharacteristic of a Li-exchanged HP type zeolite.

Although this invention has been illustrated by reference to specificembodiments, it will be apparent to those skilled in the art thatvarious changes and modifications may be made which clearly fall withinthe scope of this invention.

I claim:
 1. A synthetic crystalline lithium aluminosilicate zeolite HP,formed by ion exchange from a sodium aluminosilicate zeolite HP having alattice constant of 25.02-25.10 Å, containing a pyridine.
 2. A syntheticcrystalline lithium aluminosilicate zeolite HP as claimed in claim 1wherein said pyridine is pyridine se or a substituted pyridine selectedfrom the group consisting of 2-picoline, 3-picoline, 4-picoline,2,4-lutidine, 2,6-lutidine, and 3,4-lutidine.